Intracranial pressure monitor

ABSTRACT

Pressure monitoring apparatus implantable in the cranium to measure intracranial pressure, the apparatus comprises a passive resonant circuit having a natural frequency influenced by ambient pressure. The resonant circuit has inductance and capacitance capability for comparing the local environmental pressure to that of a volume of gas trapped inside the apparatus, the environmental pressure being measured by observation of the frequency at which energy is absorbed from an imposed magnetic field located externally of the cranium.

STATEMENT OF GOVERNMENT INTEREST

The invention described herein was made in the course of work under agrant or award from the Department of Health, Education, and Welfare.

BACKGROUND OF THE INVENTION

A. Field of the Invention

The invention finds utility for monitoring intracranial pressure indiagnostic and post-operative situations, the pressure-sensitiveapparatus of the invention being totally implantable in the cranium.

B. Description of the Prior Art

The need for monitoring intracranial pressure has long been recognizedfor applications involving intracranial hypertension. Although such aneed is well-identified for hydrocephalic individuals and individualswho have undergone neurosurgery, other critical situations involveindividuals subject to brain swelling, edema, obstruction of cerebralspinal fluid pathways, or intracranial space-occupying lesions. Accuratemonitoring of the intracranial pressure in certain of these situationsallows institution of emergency procedures should pressure rise todangerous levels.

Common methods for measuring intracranial pressure involve implantationof a pressure transducer having wires which pass through the skull andscalp. Measurement of the pressure of the cerebral spinal fluid, whichcan be related to the intracranial pressure, has generally involvedlumbar puncture or introduction of a catheter into the ventricularspaces. None of these techniques are suitable for prolonged measurementof these pressures. Danger of infection, patient discomfort, and thecertain need for a second operation to remove the measuring device arenegative aspects of all of these prior art techniques. Certain of thesetechniques actually cause leakage and blockage of the hydraulic systemwithin the cranium and directly affect pressure measurements.

A number of intracranial pressure measurement systems have beenpostulated and even tested in recent years. Virtually all of thesesystems involved placement of a transducer within the cranium with wirespassing through the scalp to a recordation sub-system. Use of thesesystems posed a constant risk of infection and required constantadjustments to compensate for changes in the position of the patient.Such systems were necessarily short-term in use. Attempts were made byAtkinson et al and Olson et al. in 1967 and 1968 respectively to implanta variable capacitor mounted on two sides of an air-filled tambour, theresonant frequency of the variable tuned circuit then being read byimposing a radio wave thereon through the intact scalp. The devices thusproposed were subject to extreme fragility and needed to be constantlyrecalibrated for temperature and atmospheric pressure changes. Further,error was prevalent in the use of these devices due to drift in the zeroreading, i. e., "baseline drift".

SUMMARY OF THE INVENTION

The invention is a system for monitoring either continuously orintermittently the intracranial pressure, the uses of which have beendescribed hereinabove. In particular, the invention provides a passiveimplantable pressure transducer useful in association with externalinterrogation and recordation apparatus for determining intracranialpressure. The present implantable pressure transducer can be permanentlyplaced in a trephine or "butt" hole in the skull and operates withoutthe need for percutaneous extracranial connections to monitoringapparatus.

Measurement of intracranial pressure gradients are necessary toanticipate and thereby effectively treat secondary complications ofcerebral insults, such as transtentorial herniations, obstructuvehydrocephalus, and rapidly expanding hematomas. The present inventionallows ready determination of specific treatment modalities to reversethese complications as well as providing useful information in thetreatment of cerebral edema of idiopathic hydrocephalus in children.Since individuals in whom intracranial pressure monitoring is mostdesirable are those in whom neurosurgical intervention is necessary oranticipated, the unavoidable requirement for a small burr hole throughthe skull is acceptable. However, unlike previously used devices, thepresent pressure transducer is operable without the need for electricalcircuits or manometric conduits which extend through the scalp, both ofwhich offer a portal of entry for infection and compromise patientmobility.

The implantable pressure transducer hereby provided comprises anon-porous yet compliant enclosure which contains a specific mass oftrapped gas and a passive r-f resonant circuit (inductance andcapacitance) having a natural frequency which is influenced by thepressure of the environment of the transducer. The capacitance portionof the r-f resonant circuit is comprised of two bellows each with oneclosed end. The closed ends, lying in close proximity to each other,form a capacitance directly proportional to their areas and inverselyproportional to their spacing. Increasing intracranial pressureelongates both bellows diminishing the spacing of the closed ends and,thereby, lowering the natural frequency of the resonant circuit. Anequivalent embodiment of this principal is achieved with one bellowshaving its closed end lying in close proximity to a fixed conductivesurface. In effect, the transducer acts to compare the surroundingenvironmental pressure to that of the gas trapped inside the transducer.

The resonant frequency of the transducer is sensed by monitoringapparatus external of the cranium by determining the natural frequencyat which the transducer absorbs energy from an electromagnetic field.This measured frequency is then converted directly to a pressurereading.

The implantable transducer is characterized by the following features:

(1) sufficient elastic compliance so that measurable deformations resultfrom small changes in intracranial pressure;

(2) non-porosity of the enclosure so that the mass of gas trapped withinthe device does not change meaningfully during the useful life of thedevice; and,

(3) electrical non-conductivity so that the resonant circuit in thedevice is not shielded from an external monitoring radio-frequencymagnetic field.

These characteristics are produced primarily by utilization of acombination of ceramic (non-porous and non-conductive) and metallic(non-porous and compliant) material to form the chamber enclosing thetrapped volume of gas within the device.

In addition to other "absolute pressure" embodiments of the invention, a"gauge pressure" embodiment of the invention is also provided in whichthe difference in pressure between the intracranial pressure and thepressure immediately beneath the scalp, which pressure is anapproximation of barometric pressure, is measured.

Accordingly, it is a primary object of the invention to provide a systemfor monitoring intracranial pressure without the need for percutaneousextracranial electrical connections or manometric conduits.

It is a further object of the invention to provide an implantablepressure transducer which passively provides an indication ofintracranial pressure to extracranial monitoring apparatus oninterrogation of the transducer.

It is another object of the invention to provide an implantable pressuretransducer having sufficient elastic compliance to be measurablydeformed by changing intracranial pressure while being sufficientlynon-porous to prevent loss of entrapped gas from the transducer orleakage of body fluids into the transducer.

It is yet another object of the invention to provide an implantablepressure transducer which provides a measure of the difference betweenthe intracranial pressure and the pressure immediately beneath thescalp, which pressure is an approximation of atmospheric, and thusbarometric, pressure.

Further objects and advantages of the invention will become more readilyapparent in light of the following detailed description of the preferredembodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustrating the environment and general operationof the invention;

FIG. 2 is a detailed elevation in partial section of an implantedtransducer according to one embodiment of the invention;

FIGS. 3a, 3b, and 3d are assembly views in perspective illustrating theseveral parts of one embodiment of the implantable pressure transducerin various stages of assembly;

FIG. 4 is a perspective in partial section of one embodiment of theimplantable pressure transducer;

FIG. 5 is a schematic illustrating in part the manner in which thetransducer is mounted in the cranium;

FIG. 6 is a graph illustrating the resonant frequency response of atypical transducer versus the sensed intracranial pressure;

FIG. 7 is an elevation in partial section of a portion of a transducerillustrating the sealing thereof;

FIG. 8 is an idealized assembly perspective illustrating the arrangementof elements used to implant the transducer;

FIG. 9 is an idealized perspective illustrating the method employed toposition the transducer properly in the cranium;

FIG. 10 is a graph illustrating the pressure response of the transducerat varying implantation depths in the cranium;

FIG. 11 is a block diagram illustrating the electrical components of thetransducer, the external detector, and the external monitoring apparatusused to detect and monitor the resonant frequency of the transducer;

FIG. 12 is an elevation in partial section of a second embodiment of thetransducer;

FIG. 13 is an elevation in partial section of a third embodiment of theinvention;

FIG. 14 is a top view in section of the embodiment of FIG. 13; and,

FIG. 15 is an elevation in partial section of a "gauge-pressure"embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention particularly provides an intracranial pressure sensingimplant, referred to hereinafter as the transducer, which contains apassive radio-frequency resonant circuit having a natural frequencyinfluenced by the pressure of the transducer's environment. Thetransducer is configured so that a comparison is continuously madebetween the environmental pressure and the pressure of a fixed mass ofgas entrapped inside the transducer. The environmental pressure iseffectively measured by observing the frequency at which the transducerabsorbs energy from an externally imposed electromagnetic field. Theinvention further provides apparatus for imposing an electromagneticfield on the transducer and for remotely measuring the frequency atwhich energy is absorbed by the transducer.

Referring now to FIGS. 1 and 2, the present implantable transducer isseen at 10 to be positioned within a burr hole 12 in the skull 14 of anindividual who has need for monitoring of the intracranial pressure. Thelower face of the transducer 10 is seen to be positioned against thedura 16, a membrane which lies beneath the skull 14 and above thesubarachnoid space, i.e., the space between the skull 14 and the brain.The transducer 10 can be mounted as will be described in detailhereinafter to bear against the dura 16 without deflecting the duradownwardly enough so as to obliterate the subarachnoid space below thetransducer. The dura 16 may therefore act to transmit subdural cerebralspinal fluid pressure to the transducer 10. Pressure sensed by thetransducer 10 is measured by an external detector 18 which is positionedexternally of the scalp over the implanted transducer 10, the detector18 being electrically connected to a recording and alarm monitor 20.Through use of the monitor 20 in a manner to be described hereinafter, acontinuous record of intracranial pressure can be made. Additionally, avisual or audio alarm can be integrated into the monitor 20 to provide asignal when intracranial pressures reach dangerously high (or low)levels.

The structure of the transducer 10 can best be seen in FIGS. 3a through3d and 4 to comprise two disc-like sections 22 and 24 which are bondedtogether to form a hollow circular cylinder. Each section has a centralaperture 26 formed in its planar face, the apertures 26 respectivelyreceiving bellows 28 and 30 thereinto. The sections 22 and 24 are formedof a ceramic or other non-porous material and are suitably bondedtogether, such as with glass frit or with epoxy cement. The bellows 28and 30 are preferably formed of gold-plated nickel. The bellows 28 and30 are each formed with an annular flange 32 surrounding the open endthereof, the flange 32 being bonded, such as by epoxy, to the peripheralportion of the planar face of each section 22 or 24 which surrounds eachof the apertures 26. The planar closed end of each of the bellows 28 and30 is disposed toward the interior of the enclosed chamber 34 formed onbonding together of the sections 22 and 24, the said closed endsextending into the chamber from opposite sides thereof and being spacedapart by a finite, pre-determined spacing as will be described ingreater detail hereinafter.

The hollow circular cylindrical chamber 34 defined by the sections 22and 24, when fully closed by bonding of the bellows 28 and 30 intorespective apertures 26, forms a reference pressure enclosure in whichis entrapped a given mass of a gas (or other compressible non-toxicfluid), the pressure of which mass of gas is used as a referencepressure relative to the pressure of the transducer's environment, i.e.,within the skull 14 in which the transducer 10 is disposed. The gaspreferably takes the form of pure nitrogen having condensable gases suchas water vapor removed therefrom. The gas within the chamber 34 issealed therein at standard atmospheric pressure. Leakage of gas from thechamber 34 or seepage of other fluids into the chamber 34 is to beprevented. Bonding of the flanges 32 of the bellows 28 and 30 to thesections 22 and 24 must therefore be accomplished so as to minimizechanges in this reference pressure gas volume. A metallization ring canbe formed around each of the apertures 36 and flange 26 solderedthereto. Alternatively, the flanges 26 can be bonded to the ceramicmaterial with epoxy. Although epoxy is a more porous material thansolder, the epoxy joints have a high ratio of path length to diffusioncross-section compared to that of the ceramic portion of the transducer10. Simple epoxy bonding is therefore considered to be adequate andeconomically attractive since "base-line drift", i.e., change in thereference pressure, is sufficiently slight with such bonding as to betolerable.

The assembled sections 22 and 24 further have an inductance coil 36wound thereabout, the ends of the coil 36 being respectively soldered tothe bellows 28 and 30 to form a passive resonant circuit in which theclosed ends of the bellows lie in proximity to each other to form thecapacitive portion of the circuit. The inductance coil 36 preferably isformed with eight turns. The coil 36 can be formed of silver, platinum,copper, or other highly conductive material. The conductive materialshould have low electrical resistance in order to provide a high Q tothe resonant circuit of which the coil 36 forms a part. Silver andplatinum would be preferred due to the less toxic properties of theseconductors relative to copper. Platinum is least toxic of the threeconductors mentioned, although the relatively higher resistance of theplatinum causes some loss of Q. An increase in pressure externally ofthe transducer 10 acts to elongate both of the bellows 28 and 30,thereby bringing the closed ends thereof closer together to increase thecapacitance and to lower the resonant frequency of the circuit. Pressurechanges can thus be monitored externally of the transducer 10 bymeasurement of the frequency at which the circuit absorbs energy from anexternally imposed electromagnetic field.

As discussed briefly hereinabove, the transducer 10 must exhibitsufficient elastic compliance such that a measurable deformation willresult from relatively small changes in intracranial pressure. At thesame time, the enclosure for the entrapped gas must be extremelynon-porous so that the mass of gas entrapped will not change appreciablyduring the useful life of the transducer 10. Further, the transducer 10must be electrically non-conductive so that the resonant circuit formedby the bellows (28 and 30) and the inductance coil 36 will be shieldedfrom the externally imposed electromagnetic field. The use of plasticmaterial for the sections 22 and 24 would be sufficiently compliant andnon-conductive but prove to be too porous. The use of non-porous andnon-conductive glass proves to be unsatisfactory due to thenon-compliant nature of the material in structurally adequate wallthicknesses. The requirement for low porosity, low electricalconductivity, and relatively high elastic compliance are met in thetransducer 10 by forming the major portions of the planar faces theenclosure (formed by the sections 22 and 24 with central apertures 26)of non-porous and non-conductive ceramic material and by forming reducedportions of the planar faces with metal, i.e., the bellows 28 and 30,the metal being non-porous and compliant. Since the metal is alsoelectrically conductive, a portion of the magnetic field absorptioncross-section of the transducer 10 is shielded. However, by controllingthe percentage of the surface area of the planar faces of the transducer10 which is subtended by the metal bellows 28 and 30, the percentage ofthe magnetic field absorption cross-section which is shielded to theexternally imposed electromagnetic field is readily held withinacceptable limits. The area of the metal portion forming the planarfaces of the transducer 10 is preferably held to approximately 20% inorder to retain sufficient elastic compliance while limiting shieldingas aforesaid. This shielding diminishes the coefficient of couplingbetween the transducer 10 and external monitoring apparatus to bedescribed, thereby reducing slightly the implantation depth at which theresonant frequency of the transducer 10 can be measured.

The transducer 10 is enclosed within a casing 38 formed of twocylindrical half-sections 40 and 42, the half-sections 40 and 42 beingbonded together to form the closed cylindrical casing 38. The casing 38is preferably formed of a suitable bio-compatible plastic such as Lexan,a product of General Electric Inc., or Polysulfone. The half section 40is seen to be formed with threading 44 over the cylindrical surfacethereof and with slots 46 in the upper planar face 43 thereof, the slots46 not extending through the face. The half-section 40 is bonded to thehalf-section 42 using a suitable solvent and plastic grit of the type ofplastic employed as the material of which the half-sections are formed.The lower planar face 45 of the half-section 42 is typically made lessthick than the upper planar face 43, the face 45 being as thin as isstructurally practical in order to aid in pressure transfer across saidlower face. For example, if the half-sections 40 and 42 are formed ofpolysulfone, a grit of polysulfone dissolved in dichloroethane is usedto cement the half-sections 40 and 42 together. Mating inner and outerannular shoulders 48 and 50 form a lap-joint 52 which seals the casing38 more securely.

The spacing internally of the casing 38 and externally of the transducer10 is filled with a suitable fluid 54, such as medical grade siliconliquid, which transmits external pressure on the casing 38 to thebellows 28 and 30. The fluid 54, in addition to its ability to transmitpressure, should be chosen to have a low dielectric constant in order tominimize stray capacitance, to have a low ability to absorb moisture,and to be non-toxic. The casing 38 acts at its most basic level toisolate the circuit formed by the bellows and the coil 36 fromconductive body fluids which would short-circuit the bellowscapacitance.

Since a number of the transducers 10 would be manufactured in order tomeet the clinical needs mentioned hereinabove. it is believed necessaryto discuss calibration and manufacturing control considerations whichare useful to a reasonable practice of the invention. Calibration of thetransducer 10 is essentially a matter of control of the spacing of theclosed ends of the bellows 28 and 30. At atmospheric pressure and atusual body temperature, the closed ends of the bellows are to be spaced0.004 inch apart. Control of the spacing between the closed ends of thebellows is principally a matter of controlling the thickness of theepoxy bond between the two ceramic sections 22 and 24. It is deemedpreferable to epxoy bond the sections 22 and 24 together rather than tofire a joint therebetween using glass frit, since epoxy bonding can bedone after the bellows 28 and 30 are mounted on the sections 22 and 24whereas firing of the glass frit requires that the heat-treatablebellows be mounted after the bonding between the sections isaccomplished. The spacing uncertainty introduced by the bonding of thebellows 28 and 30 to the ceramic sections 22 and 24 can be determinedelectrically and eliminated by honing the ceramic sections on a diamonddust-impregnated copper flat. Honing is performed on the surfaces of thesections 22 and 24 which are later bonded together.

Control of the reference pressure within the sealed transducer 10requires the epoxy bonds between the ceramic sections 22 and 24 to beformed and cured, such as by baking, before final closure of the chamber34. A small vent hole 56 formed in one of the sections, such as in thesection 22, is used to seal the chamber 34 in a manner to be describedmore fully hereinafter. Compensation for error in spacing between theclosed ends of the bellows 28 and 30 requires a bias in the closuretemperature at the rate of 10.8° C. per milli-inch of spacing error. Theintegrity of the bonded joints must also be insured since even a minuterate of leakage into or from the chamber 34 results in an unacceptablerate of drift of the reference pressure. Thus, the chamber 34 is leaktested after closure of the vent hole 56 and allowance for the entrappedgas to reach room temperature. As the entrapped gas in the sealedchamber 34 cools to room temperature, the resonant frequency of thecircuit diminishes approximately 8.4 MHz. If any gross leaks arepresent, the frequency will drift back to the body temperature value. Ifthe room temperature frequency remains stable, indicating no grossleaks, the transducer 10 is then fine leak tested by soaking thetransducer in pressurized helium for a period of time to allow helium toenter through any microscopic leak which might exist. The transducer 10is then placed in a helium leak tester vacuum chamber to detect escapeof helium for the chamber 34. If no helium is detected, the transducer10 is considered to be leak free.

The operation of the transducer 10 can be described by the followingmathematical relationships, the pressure P_(o) external of thetransducer being related to the pressure P_(i) by the following:##EQU1## where:

k= the spring constant of the bellows 28 and 30;

A= cross-sectional area of the bellows closed end;

X_(o) = neutral (unstressed) position of the bellows (half theseparation of the bellows); and,

x= the stressed position of the bellows.

Since the volume change caused by bellows deformation is negligible, theinternal pressure P_(i) can be expressed in terms of temperature asfollows: ##EQU2## where P_(c) and T_(c) are respectively the closurepressure and temperature at the time the vent hole 56 in the transducer10 is sealed. The ideal value for these last two parameters are 10,336mm H₂ O (760 mm Hg) and 558°R (body temperature, absolute). Hence thetemperature sensitivity of the transducer 10 is 18.5 mm H₂ O/F°. and theformula giving the pressure correction (P_(T)) for body temperature(T_(B)) is:

    P.sub.T =18.5 (T.sub.B -98.6)

the distance (X in milli-inches) of the surface of the bellows 28 and 30from the midpoint between the bellows can be given in terms of theresonant R-F frequency (ω): ##EQU3##

L= inductance (1.4× 10.sup.⁻⁶ henries);

K= a constant (4.039); and,

C_(s) = stay capacitance (μμF).

The general equation can now be written: ##EQU4## where:

P_(BP) = barometric pressure (mm H₂ O);

P_(a) = applied pressure (mm H₂ O);

T= temperature (F. degrees absolute) ##EQU5##

= 2π)² LC_(S) ; and

f_(o) = frequency under standard conditions of pressure 10,366 mm H₂ O)and temperature.

The last three numbers γ, β, and f_(o) are sufficient to describecompletely the performance of the transducer 10. The value for f_(o) ismeasured directly and the values for γ on β are determined from twofrequency readings f₁ and f₂ at two different pressure readings P₁ andP₂ made at a known temperature and barometric pressure. The frequencyresponse of the transducer 10 versus the pressure acting on thetransducer 10 is shown graphically in FIG. 7.

Referring now to FIGS. 1, 2, and 5, the transducer 10 encased in thecasing 38, which will be referred to hereinafter as the "encasedtransducer 10", is seen to be held within a cranial burr hole 12 bymeans of a collar 58. Implantation of the transducer 10 can be madeunder local anesthesia through a curvilinear incision in the scalp 64.The burr hole 12 is made in the skull 14, usually as the result oftreatment for a siutation which incidentally requires monitoring ofintracranial pressure, according to known procedures: Typically, anair-driven trephine or brace trephine is used to make the hole 12, thecavity in the skull 14 being trimmed with a curette to expose a circulararea of dura. Bone wax is used to stem bleeding from the walls of theburr hole 12 and a bipolar coagulator is used to stop any bleeding whichmay exist on the surface of the exposed dura 16. The collar 58 is thenattached to the skull 14, an annular flange 60 on the upper end of thecollar resting on the table of the skull 14. The flange 60 hasperipherally spaced apertures (not shown) therein which allows suturingof the collar 58 to the periosteum or bone. In the event the periosteumis not available, the galea tissue is turned over the flange 60 as aflap and secured with interrupted sutures. The scalp margins areapproximated for subsequent wound closure. An annular cylindrical neck62 which forms the remaining portion of the collar 58 extends into theburr hole 12 and is threaded internally at 63 to receive the threads 44on the outer cylindrical surface of the casing 38 of the encasedtransducer 10. The encased transducer 10 can thereby be screwed into theneck 62 of the collar 58 to a depth sufficient to cause the lower planarface 45 of the casing 38 to bear against the dura 16 as seen clearly inFIG. 2. The encased transducer 10 is rotated within the collar 58 bymeans of a two-pronged wrench, shown in FIGS. 8 and 9 to be describedhereinafter, which mates with the slots 46 in the upper planar face 43of the half-section 40. The scalp 64 is then closed over theimplantation thus formed. It should be understood that the encasedtransducer 10 can be implanted in a number of other ways. For example,the encased transducer 10 can be simply placed in the burr hole 12 andpushed to one side thereof between the dura 16 and the skull, the burrhole 12 then being sealed with a plug or with bone wax. Alternatively,the encased transducer 10 can be implanted subdurally. According to thepreferred manner of implanting the encased transducer 10 as describedabove and as will be further described hereinafter, the dura 16 is notdeflected downwardly by the lower planar face 45 of the casing 38sufficiently to obliterate the subarachnoid space 66 below the casing 38between the dura 16 and the brain 68. The dura 16 can then act as asecond diaphragm to transmit the subdural cerebral spinal fluid pressureto the encased transducer 10.

As has previously been briefly described, the external detector 18 isbrought into spaced relationship to the implanted transducer 10 onclosure of the scalp 64. The detector 18 is usually taped on thesurgical dressing which overlies the site of the implanted transducer10. As will be soon described in detail, the detector 18 interrogatesthe implanted transducer 10 by directing electromagnetic energy into thetransducer, the frequency at which the transducer absorbs the incidentelectromagnetic energy being a measure of the intracranial pressuresensed by the transducer. The detector 18 is connected to the monitor 20as aforesaid, the monitor 20 operating continuously to provide apermanent record of the intracranial pressure. The monitor 20 can takethe form of medical monitoring apparatus now commercially available.Desired functions which are within the art can be readily incorporatedinto the monitor. For example, visual or auditory alarms can be causedto operate if the sensed pressures reaches certain predetermined levels.The monitor 20 can be designed, however, so that it will not respond tomomentary increases in pressure such as are typically brought about bycoughing or straining. Since the transducer 10 essentially acts as asmall barometer and is thereby responsive to absolute pressure,correction for barometric pressure can be included in the monitor 20itself. Alternatively, corrections can manually be made in response tobarometric pressure changes.

Prior to implantation, the encased transducer 10 (as well as the collar58) is to be sterilized internally as well as externally. The transducer10 cannot be autoclaved because the great heat would rupture the bellows28 and 30. While the encased transducer 10 can be brought to atemperature of 120° C. and held at that temperature for an extendedperiod of time, sterilization by radiation appears to be the bestprocedure.

Referring now to FIG. 7, a preferred method for sealing the transducer10 is shown. As aforesaid, a vent hole 56 is formed in one of theceramic sections 22 or 24. After assembly of the bellows 28 and 30 tothe sections and bonding together of the sections, the specific mass ofgas is finally entrapped within the chamber 34 by sealing of the venthole 56. The vent hole 56 is best sealed by providing a thin annularmetal insert 57 within the hole 56. The insert 57 has sloping innercavity walls and may preferably be formed of brass. The insert 57 can besimply coated with soft solder while leaving the hole 56 open. Thetransducer 10 would then be brought to body temperature and the soldercoating touched with a hot iron to close the hole 56. However, it isbelieved preferable to first insert a pin 70 into the hole 56 as shownin FIG. 7. Solder 72 is then applied to the sides of the pin 70, thesolder 72 adhering also to the metal insert 57 to seal the hole. The pin70 has sloping surface walls which mate with the sloping walls of thecavity in the insert. The transducer 10 is brought to a desiredtemperature, i.e., body temperature modified by 0.42° C. per millimeterof mercury atmospheric pressure deviation from standard. The use of thepin 70 prevents gaseous elements from the solder from diffusing into thechamber 34, thereby altering the reference pressure provided by the gasentrapped therein. The pin 70 can be severed just above the solder jointafter sealing.

The method of implantation of the transducer 10, as described relativeto FIGS. 1, 2, and 5, can be seen in further detail through theillustration provided by FIGS. 8 and 9. In FIG. 8, a subject is shownwith the collar 58 sutured to the skull 14, the transducer 10 initiallyhaving been manually threaded into the collar 58. As seen in FIGS. 8 and9, a tool 74 having spaced pins 75 which mate with the slots 46 in thecasing 38 is used to rotate the encased transducer 10 further into thethreaded collar 58 toward contact with the dura 16 as aforesaid. Inorder to determine when full contact occurs between the lower planarface 45 of the casing 38 and the dura 16, the detector 18 is providedwith a central aperture 76, inductance coils which are a part of thecircuitry of the detector 18 and which will be described hereinafterlying about the periphery of the central aperture 76. The detector 18 isplaced over the encased transducer 10 and the collar 58, the aperture 76being sufficiently large to allow the tool 74 to be fitted into contactwith the encased transducer through the aperture 76. The tool 74 isrotated to cause the encased transducer 10 to be seated more deeplywithin the threaded collar 58. While the tool 74 is thus being operated,the detector 18 is used to monitor the pressure sensed by the encasedtransducer 10. Referring now to FIG. 10, the pressure sensed by thetransducer 10 is shown versus the depth of the transducer within thecollar 58. The pressure sensed by the encased transducer 10 remainsrelatively constant until the lower planar face 45 of the casing 38initially contacts the dura 16, at which time the sensed pressureincreases abruptly as seen at 77. As the encased transducer 10 isfurther moved toward the dura 16, the sensed pressure first continues toincrease and then levels off to form a plateau at 78 in thepressure-depth curve of FIG. 10. This plateau indicates that fullcontact between the lower planar face 45 of the casing 38 and the dura16 has been achieved. Substantial additional movement of the encasedtransducer 10 in an inward direction eventually causes a second abruptincrease in pressure, as seen at 79, this pressure increase indicatingthat the subarachnoid space between the dura 16 and the brain has beenobliterated, the encased transducer 10 therefore having been insertedtoo deeply into the cranial cavity. The tool 74 could therefore be usedto withdraw the transducer 10 outwardly within the collar 58 to thedesired depth as shown by the re-establishment of the plateau 78 on thedepth-pressure curve of FIG. 10. In practice, once the plateau 78 isestablished during simultaneous insertion of the encased transducer 10and monitoring thereof through use of the detector 18, the transducer 10is considered to be located properly and the tool 74 and the detector 18are removed. The wound is then closed as described hereinabove andintracranial pressure is monitored externally through use of thedetector 18 and the monitor 20.

As indicated above, the transducer 10 contains a passive resonantcircuit formed by the bellows 28 and 30, which act as a capacitor, andthe coil 36, which acts as an inductor, the circuit having a high Q. Thenatural frequency of the circuit is influenced by the pressure "seen" bythe encased transducer 10, the pressure of the gas trapped within thechamber 34 being compared to this environmental pressure. An increase inthis environmental pressure, i.e., the intracranial pressure, causes thebellows 28 and 30 to elongate to bring the closed ends of said bellowscloser together, thereby increasing the capacitance of the resonantcircuit and, accordingly, lowering the r-f resonant frequency of thecircuit. Measurement of any change in the r-f resonant frequency of theresonant circuit is made through the scalp 64 (and medical dressings) byuse of the detector 18 and monitor 20. The detector 18 is placed overthat location on the scalp which surmounts the implanted transducer 10.The resonant frequency of the transducer circuit is then determined bysubjecting the circuit to a frequency swept RF signal from the detector18, the frequency at which the electromagnetic energy is mostefficiently coupled into the transducer circuit, wherein said energy isdissipated by resistive losses, then being detected by the detector 18.

A well-known device such as a grid-dip oscillator (not shown) can beemployed to interrogate the implanted transducer 10, the oscillatorbeing used with associated means for varying frequency over a rangewhich includes the resonant frequency of the transducer circuit andadditional means for identifying the frequency at which a grid currentdip occurs. Such means are known in the art. A grid-dip oscillatoressentially is itself a resonant circuit, i.e., a circuit with high Q,which consists of a capacitor and an inductance coil in an oscillatorcircuit. However, this well-known device has poor output regulation, theoscillation amplitude being markedly diminished by even slight loading.Further, such a device must be continuously tuned manually and does notdisplay dynamic pressure changes.

The transfer of energy by inductive coupling from an inductor externalof the cranium, i.e., in the detector 18, to the inductance coil 36 inthe transducer 10 can be more efficiently and effectively accomplishedthrough use of the system shown in FIG. 11. The detector 18 is seen tobe electrically joined to the monitor 20 by means of a single coaxialtransmission line 80 which conveys RF signals from the monitor 20 to thedetector 18 and simultaneously returns the signal detected by detector18, which detected signal appears as a DC voltage, to the monitor 20.Although separate coaxial cables could be used to separately convey theRF input and the DC output, the presence of two separate cables attachedto the detector 18 results in reduced flexibility of the combination,the detector 18 thereby being less handy to use for routine monitoring.

The RF input to the detector 18 typically has a characteristic impedanceof 50 ohms, the signal being driven by a swept frequency signal sourcein the monitor 20 which has an output impedance of 50 ohms. A 50 ohmsresistor 81 is present in the circuit of the detector 18, the resistor81 minimizing the SWR over the operating frequency band so thatreflections do not cause extraneous frequency dependent peaks and nulls.A parallel resonant LC circuit comprised of capacitor 82 and inductor 83is placed across the transmission line 80. The bandwidth of thisresonant circuit is intentionally made large by using a large L to Cratio along with the heavy resistive loading caused by the resistor 81in parallel with the 50 ohm RF source resistance, inductor losses beingcomparatively negligible. The inductor 83 is physically constructed andpositioned in the detector 18, to maximize inductive coupling with theinductance coil 36 in the implanted transducer 10. The inductor 83essentially takes the form of a flat ribbon spiral as seen on thesurface of the detector in FIGS. 8 and 9. A diode 84 detects the RFsignal appearing across the inductor 83 and produces a DC voltage outputwhich is proportional to the existing RF voltage. The diode 84 ispositioned in the detector circuit so that this resulting DC voltageappears on the RF input transmission line. A low RF impedance couplingcapacitor 85 passes the RF signal to the resistive termination and tothe broadband resonant LC network. The capacitor 85 also blocks the DCvoltage signal from being short circuited to ground through the inductor83.

In normal operation the RF signal source in the monitor 20 (to bedescribed in detail hereinafter) is swept linearly from 50 MHz to 100MHz at a 60 Hz rate and provides a constant output power level of about1 milliwatt (OdBm). The output power level is preferably maintained at aconstant level over the sweep range to within ± 0.25 dB. In the absenceof inductive coupling, the RF voltage at the detector 18 remainsconstant throughout the sweep range. The resulting DC output voltage islikewise constant.

When the resonant circuit of the implanted transducer 10 is coupled tothe LC circuit of the detector 18, energy is coupled from the detector18 to the transducer 10 only at the resonant frequency of thetransducer. When this energy transfer occurs, an accompanying reductionin the RF voltage appears across the inductor 83. This reduction can beenvisioned as either a power transfer which loads down the source of theRF voltage or, equivalently, as an impedance transformer which reflectsa low value resistnce across the inductor 83. The magnitude of the RFvoltage drop is dependent primarily on the amount of inductive couplingwhich is maximized when the two inductors, i.e., the coil 36 and theinductor 83, are positioned coaxially in the closest possible proximity.The loading effect occurs only at the resonant frequency of thetransducer 10, consequently, as the applied RF signal is swept acrossthe operating band, the circuit of the detector 18 will respond to a dipin RF voltage across the inductor 83 as the swept signal passes throughthe resonance of the transducer 10. The bandwidth of this dip isdetermined jointly by the Q of the resonant circuit of the transducer 10and by the amount of inductive coupling. Low Q and tight couplingincrease bandwidth and impair resolution of the resonant frequency. Bycorrelating the occurrence of the voltage dip with the RF frequency ofthe source of sweep generation, the frequency of resonance isdetermined, the intracranial pressure being related thereto.

In order to recover the DC voltage signal from the transmission line 80and to prevent said signal from being loaded by the RF signal source, abias tee 86 is provided in the transmission line 80, the bias tee 86being integrated into the monitor 20. The bias tee 86 receives the RFvoltage signal through a 10 db attenuator 87 from a voltage controlledRF sweep oscillator 88. The bias tee 86 essentially consists of a low RFimpedance coupling capacitor 89 which passes the RF voltage signal fromthe sweep generator 88 and blocks the DC voltage signal returning fromthe detector 18. The DC voltage signal is shunted through a radiofrequency choke 90 in the bias tee 86, the choke 90 appearing as anegligible high impedance to the RF signals while passing the DC signalto a DC amplifier 91. A low RF impedance feed through capacitor 92positioned between the choke 90 and the amplifier 91 acts to prevent RFsignals from being passed on to the amplifier 91. The amplifier 91 cantypically provide a 10,000 ohm DC load to the detector 18 and therebyproduces an inverting DC gain of 20. The DC output of the detector 18 istypically on the order of -50 mV, thereby producing a +1.0 volt outputfrom the amplifier 91. Typical components values in the bias tee 86 are2,000 pf for the capacitor 89, 2.2μ h for the RF choke 90, and 1,000 pffor the capacitor 92. In the detector 18, typical component values are1,000 pf for the capacitor 85, 51 ohms for the resistor 81 and 10 pf forthe capacitor 82. The diode 84 can be a Schottky barrier type, asuitable inexpensive commercial version of which is the HP 5082-2800.

The inductor 83 takes the form of a spiral inductance coil as seen inFIGS. 8 and 9, which coil can be formed on a flat PC-type board. In sucha typical construction, the inductor 83 would exhibit an inductance of0.305 microhenrys and would have a self resonant frequency of about 270MHz, thereby indicating a stray capacitance of about 1.1 picofarad. Whenusing an additional 10 pf capacitor, the circuit of the detector 18resonates at about 87 MHz, such resonance providing a nearlysymmetrical, slightly rounded response from 50 MHz to 100 MHz with onlya slight midband peak.

The monitor 20 is configured to drive a commercial X-Y displyoscilloscope 93, such as the Telonic Model 121 or the Wavetek Model1901A. The monitor 20 itself can consist primarily of a commerciallyavailable RF sweep generator such as the Wavetek Model 1050A sweepgenerator, this commercial unit being modified in several ways.Primarily, this commercial sweep generator is modified by adding thebias tee 86 for extraction of the DC voltage signal from the detector 18and by adding the DC amplifier 91 for increasing the DC signal.Frequency marker signals to be described hereinafter are also added toassist in signal processing. Other functions added to this commercialsweep generator include: (1) restriction of the frequency range of thecenter frequency control to a desired band; (2) replacement of the RFoutput step attenuator with the 10 db attenuator 87 to set the outputlevel at 0 dBm; (3) replacement of the RF output vernier control with afixed setting; (4) removal of the demodulator input connection; (5)re-positioning, calibration, and limiting the range of the sweep widthcontrol; (6) replacement of the marker width control with a fixed value,(7) rewiring of the AC power switch to break both sides of the line;and, (8) fabrication of a control panel with controls labeled andpositioned to facilitate use of the monitor 20.

The monitor 20 is then configured as specifically seen in FIG. 11, thesweep oscillator 88 providing an RF output restricted to the frequencyrange of approximately 50 MHz to 100 MHz. An output level of about + 10dBm is controlled and maintained at a constant level across the sweeprange of the oscillator 88 by an internal active leveling circuit (notshown). A frequency control voltage which is fed into the oscillator 88is obtained from ramp generator 94, the ramp from the generator 94 beingshaped to compensate for nonlinearities in the oscillator 88 so that alinear frequency sweep results. Sweep rate is fixed at the AC linefrequency (60 Hz). Center frequency of the RF sweep is operatoradjustable by a center frequency variable resistor 95 over a range from50 MHz to 100 MHz. The operator would typically set the center frequencyto nearly coincide with the resonant frequency dip displayed on thedisplay oscilloscope 93. The sweep width (or frequency span) is alsooperator adjustable by a sweep width variable resistor 96 to a nominalrange of 1 MHz to 50 MHz. The operator can adjust the sweep width toobtain the desired horizontal display "magnification" to facilitateresolution of the resonant dip frequency. The resulting sweep voltage isramped symmetrically both up and down. During the down or retrace sweep,the output amplifiers (not shown) of the oscillator 88 areelectronically switched off in order to remove the RF output. A zerooutput reference is thus produced during this retrace interval andappears as a straight baseline on oscilloscope 93. The ramp generator 94also provides a linear ramp (triangular) voltage which is fed to theoscilloscope 93 to be used as the horizontal drive signal.

The leveled RF output from the sweep oscillator 88 is attenuated by theattenuator 87 to about 0 dBm. At this drive level, the transfercharacteristic of the detector 18, i.e., DC out vs. RF power in, beginsto become less exponetial, i.e., more linear. Increasing the drive levelwould also increase the amplitude of the resonance dip of the detector18. However, the average DC output would correspondingly increase in asignificantly greater fashion, thereby resulting in the magnitude of thedip being proportionately smaller in relation to the DC level away fromresonance. The summing DC amplifier 91 combines the DC output from thebias tee 86 with frequency marker output signals generated by aselectable comb marker generator 97. Demodulated signals from thedetector 18 are inverted and amplified by 20 while the marker signalsare inverted and amplified by a factor of unity. The resulting signal isfed from the amplifier 91 to the oscilloscope 93 as the vertical inputthereto. The marker generator 97 consists of three separate combfrequency generators (not shown), any one or combination of which can beselected by an operator through use of a marker size variable resistor98. The comb frequency generators are used to determine the frequency atwhich the resonant dip appears on the display oscilloscope 93. The combgenerators produce crystal controlled marker signals having 1 MHz, 10MHz, or 50 MHz spacing. These marker signals are compared with a sampleof the swept RF signal through connection to the oscillator 88. When anaudio beat frequency is produced by the two signals, the markergenerator 97 produces a clipped output marker signal.

In order to protect the patient from electrical shock hazards, themonitor is isolated from AC power lines by the use of isolationtransformers (not shown) in the power supplies. The detector 18 is alsoinsulated. Automatic correction for barometric pressure and patienttemperature can also be provided in the monitor 20 in order to produce adirect digital read-out of actual pressure without the need for manualconversion. Such correction would be implemented by a microprocessorwhich would read in data from a barometric pressure transducer, such asa National Semiconductor LX3701A or LX3801A; decode a manually fedreading of the patient's temperature; perform the frequency counterfunction to determine the frequency of the variable marker when alignedwith the resonant dip (or generate the marker signal with synthesizertechniques); and execute a stored program using this data withpre-determined parameters of the implanted transducer 10 to obtain adigital read-out of intracranial pressure.

The transducer 10 may be configured otherwise than is describedhereinabove. Referring now to FIG. 12, a cylindrical ceramic enclosure100, which can be formed in essentially the saame manner as the ceramicportion of the transducer 10, is seen to have a central aperture 102 inonly one planar face thereof. The aperture 102 has a metal bellows 104bonded therein with the closed end of the bellows 104 extending into thechamber 106 essentially defined by the ceramic enclosure 100. Oppositethe closed end of the bellows 104 on the inner wall of the upper planarface of the enclosure 100 is a flat pad 108 of metal, the pad 108 facingthe planar closed end of the bellows 104. An inductance coil 110 iswound about the enclosure 100 in the same manner as has been describedrelative to the transducer 10, the coil 110 being electrically joined atits respective ends to the bellows 104 and to the metal pad 108. Themetal pad 108 and the closed end of the bellows 104 are spaced apart apre-determined distance in the manner that was previously describedrelative to the fabrication of the transducer 10. Thus, a circuit isestablished which consists of the metal pad 108, the bellows 104, andthe inductance coil 110, the circuit essentially being a passive "LC",circuit, i.e., a circuit having an inductance and a capacitance, in themanner of the circuit formed by the bellows 28, 30 and the inductancecoil 36 of the transducer 10. In the embodiment shown in FIG. 12, one ofthe bellows has been eliminated, thereby providing a more simplestructure. The transducer thus formed would be encased for implantationand utilized essentially in the same fashion as the transducer 10.

Referring now to FIGS. 13 and 14, another embodiment of the inventioncan be seen to utilize only one bellows 120, the bellows 120 beingbonded within aperture 122 formed in one planar face of a cylindricalceramic enclosure 124. The ceramic enclosure 124 is formed inessentially the same manner as is the ceramic portion of the transducer10 as aforesaid. The opposite interior wall of the upper planar face ofthe enclosure 124 has a flat pad 126 of metal formed thereon, the pad126 being spaced from and oppositely facing the closed end of thebellows 120. On the aforesaid interior wall on which the pad 126 isdisposed, an inductance coil 128 is formed such as by chemical milling,vacuum deposition, or the like. The coil 128 preferably takes the formof a spiral, electrically connecting to the pad 126 at its inner end andextending along the interior of the enclosure 124 to connect with thebellows 120 at its outer end. The spiral coil 128 can have any number ofturns which provides a reasonable Q value to the circuit thus formed bythe pad 126, the bellows 120, and the coil 128, the coil 128 being shownwith twelve turns as a typical example. The coil 128 is shown as aspiral since this geometric figure provides a continuous electrical pathbetween the turns of the coil. Other geometrical patterns wherein theturns of the coil 128 are electrically continuous therebetween areessentially equivalent and therefore suitable. The inductance of thespiral coil 128 is clearly not as great as a coil of the same number ofturns in which all of the coils subtend the same area as is the casewith the coils 36 and 110 described hereinabove. However, the ease withwhich the coil 128 and pad 126 can be formed, coupled with the use ofonly one bellows 120, causes the structure shown in FIGS. 13 and 14 tobe an economically attractive embodiment of the present invention.

The transducer structures described above can all be used with theexternal detector 18 and monitor 20. These transducer structures measureabsolute pressure and are thus subject, as foresaid, to changes in bodytemperature, barometric pressure, and/or changes in the referencepressure levels within the transducers. Corrections for one or more ofthese factors are required at times. The embodiment of FIG. 15 providesa transducer structure which measures "gauge" pressure, or thedifferential pressure between atmospheric pressure and the intracranialpressure seen by the transducer. Correction need not be made during useof this embodiment of the invention for temperature or barometricpressure. Further, drift of the reference pressure within the transducerof FIG. 15 is also effectively immaterial to its use since only adifferential pressure is measured. Referring now to FIG. 15, atransducer 150 is seen to be comprised of a cylindrical ceramicenclosure 152 having a central aperture 154 formed in one planar facethereof. A metal bellows 156 is bonded within the aperture 154 with theclosed end of the bellows 156 extending into a chamber 158 essentiallydefined by the ceramic enclosure 152. A metal pad 160 is formed on theinner wall of the upper planar face of the enclosure 152, the metal pad160 being spaced from and oppositely facing the closed end of thebellows 156. An inductance coil 162 is wound about the enclosure 152 inthe same manner as has been described relative to the transducer 10, thecoil 162 being electrically joined at its respective ends to the bellows156 and to the metal pad 160. The extension of the conductive coil whichleads to the metal pad 160 can pass through a vent hole 164, which alsohas another function, or it can pass through a separate apertureprovided solely for that purpose. The metal pad 160 and the closed endof the bellows 156 are spaced apart a pre-determined distance in themanner that was previously described relative to the fabrication of thetransducer 10. Thus, a circuit is established which consists of themetal pad 160, the bellows 156, and the inductance coil 162, the circuitessentially being a passive LC circuit.

In effect, the transducer 150 as shown in FIG. 15 is physically similarto that structure shown in FIG. 12. It is to be understood, however,that the essential physical structure shown in FIGS. 13 and 14 couldalternatively be substituted for the transducer 150. It is to beunderstood that the bellows 156 could be disposed to communicate witheither of the chambers 166 or 170, but not with both. The vent hold 164would then communicate the chamber 158 with whichever chamber 166 or 170which does not communicate with the interior of the bellows 156. Animportant aspect of the practice of this embodiment of the invention isthe provision of the vent hole 164 in the ceramic enclosure 152 whichallows communication between the chamber 158 and an outer chamber 166which surmounts the transducer 150. The transducer structures of FIG. 12and FIGS. 13 and 14 would be so configured if substituted into thisembodiment of the invention. The chamber 166 is defined by the upperportion of a casing 168, the lower portion of the casing 168 defining alower chamber 170, the chambers 166 and 170 being sealed, such as at 172and 174, to prevent communication therebetween. A fluid 176, such assilicone oil, is disposed within the chambers 166 and 170. Since thechamber 166 and the chamber 158 of the transducer 150 communicatethrough the vent hole 164, the fluid 176 fills both chambers 158 and166. The fluid 176 within the lower chamber 170 also fills the interiorof the bellows 156. However, the fluid 176 does not communicate betweenthe chambers 166 and 170.

The casing 168 has upper and lower planar faces 178 and 180, the upperplanar face 178 essentially is responsive to atmospheric pressure whenimplanted beneath the scalp such as shown in FIGS. 1 and 2, the pressureimmediately beneath the scalp being the same as or an approximation ofatmospheric pressure. The pressure within the chambers 166 and 158 beingtherefore related to atmospheric pressure. The lower planar face 180 ofthe casing 168 is brought into contact with the dura as describedhereinabove so that the pressure within the lower chamber 170 is relatedto the intracranial pressure. The pressure within chambers 158 and 166and the pressure within chamber 170 are transmitted through fluid 176 tocause the closed end of the bellows 156 to assume a position relative tothe metal pad 160 which is a function of the difference between theatmospheric pressure and the intracranial pressure. Thus, through use ofthreads 182 on the outer cylindrical faces of the casing 168, thestructure of FIG. 15 can be implanted in the same manner as can thetransducer 10, i.e., such as within the threaded collar 58. Since theupper planar face 178 of the casing 168 should be formed with a minimumthickness consistent with structural integrity in order to facilitatepressure transfer across the face, slots 184 are preferably formed inthe outer rim 186 of the casing 168 rather than in the planar face 178.A spanner wrench or the like can then be used to rotate the threadedcasing 168 within a mounting collar such as the collar 58. The lowerplanar face 180 is also formed with a minimum thickness in order tofacilitate transfer of pressure across the face. The rim 186 is formedto extend upwardly above the level of the upper planar face 178 in orderto prevent pressure-causing impingement of the scalp on the face 178. Aperforated shroud (not shown) could also be placed in surmounting spacedrelation to the face 178 to prevent actual contact between the face 178and the scalp.

The structure of FIG. 15 can be externally monitored such as by theexternal detector 18 and monitor 20 described previously. The signalthus generated would not need either manual or automatic correction fortemperature or barometric pressure. More importantly, drift of thereference pressure due to changes in gas volume, such as in the chamber34 of the transducer 10, which changes can occur for a variety ofreasons both related to and unrelated to leakage to or from the chamber34, need not be considered with the structure of FIG. 15 since "gauge"pressure is measured directly by this embodiment without the need forpressure or temperature corrections.

The enclosure 152 can be formed of ceramic or other refractory materialin order to provide structural stiffness. Other suitable materials couldinclude structurally stiff plastics, etc., which are non-toxic and havea low dielectric constant. The enclosure 152 can be fitted into anannular race 188 to maintain said enclosure at a desired location withinthe casing 168. The chambers 177 and 170 can then be conveniently sealedfrom each other at one or both of the shoulders of the race 188. Most ofthe fabrication techniques described above relative to the encasedtransducer 10, such as epoxy bonding of the casing, can also apply tofabrication of the transducer 150.

As should be apparent from the foregoing, the invention can be practicedother than as specifically described hereinabove without departing fromthe scope and intent of the invention. In particular, the circuit formedwithin the transducers could be configured to contain a capacitanceacross the inductance portion of the circuit. Such a circuit wouldresult from the simple expedient of not providing an electricalconnection between the bellows. A circuit so formed would still comprisea resonant circuit having a large Q and which could be monitored in amanner similar to that described hereinabove. Further, only a singlebellows could be used in the transducer 10. It is therefore apparentthat the invention is to be limited only by the definition provided bythe appended claims.

What is claimed is:
 1. Apparatus for sensing pressure within a cavity inthe body of a living entity, comprising:means deformable in response topressure within the cavity, said means comprisinga housing formed ofnon-porous and electrically non-conductive material, the housingdefining a chamber, a predetermined mass of fluid disposed within thechamber, and, pressure responsive means carried on the housing andextending into the chamber, said pressure responsive means being formedof non-porous and elastically compliant material and being deformable bypressure imposed on the apparatus to change the volume occupied by themass of fluid within the chamber; and circuit means for absorbingelectromagnetic radiation imposed thereon from externally of the circuitmeans at frequencies indicative of the deformation of thefirst-mentioned means.
 2. The apparatus of claim 1 and furthercomprising:means housing the first-mentioned means and the circuitmeans, the housing means being totally implantable within the aforesaidcavity; means for imposing electromagnetic radiation on the circuitmeans within the implanted housing means; and, means for detecting thefrequency at which the circuit means most efficiently absorbs theimposed electromagnetic radiation.
 3. The apparatus of claim 2 andfurther comprisingmeans for converting the detected frequency to acorresponding pressure reading and for displaying the pressure reading.4. The apparatus of claim 2 wherein at least portion of the imposingmeans and the detecting means are disposed on a mounting substrate, thesubstrate having an aperture therein for receiving discrete tool meansfor positioning the housing means at a desired location within theaforesaid cavity.
 5. The apparatus of claim 1 wherein the fluidcomprises nitrogen gas.
 6. The apparatus of claim 1 and furthercomprising at least two electrically conductive surfaces disposed inopposing relation to each other and movable relative to each other inresponse to deformation of the pressure responsive means, the surfacesforming at least a capacitive portion of the circuit means.
 7. Theapparatus of claim 6 and further comprising inductive means disposed inproximity to the electrically conductive surfaces, the, inductive meansforming an inductive portion of the circuit means.
 8. The apparatus ofclaim 1 wherein the inductive means comprise a coil of electricallyconductive material.
 9. The apparatus of claim 8 wherein the coil iselectrically connected to each of the electrically conductive surfacesto form a resonant circuit.
 10. The apparatus of claim 1 wherein thepressure responsive means comprise at least one bellows formed of anelectrically conductive material and having a substantially planar endportion extending into the chamber, the apparatus furthercomprising:electrically conductive means disposed within the chamber andhaving a substantially planar surface opposing the planar end portion ofthe bellows, said surface and said planar end portion being movablerelative to each other in response to deformation of the bellows, thesurface of the electrically conductive means and the planar end portionof the bellows forming at least a capacitive portion of the circuitmeans.
 11. The apparatus of claim 10 wherein the electrically conductivemeans is comprised of a second bellows and the substantially planarsurface thereof is an end portion of the second bellows.
 12. Theapparatus of claim 10 and further comprising a coil of electricallyconductive material, the coil being electrically connected to thebellows and to the electrically conductive means, the coil forming aninductive portion of the circuit means.
 13. The apparatus of claim 12wherein the coil is wrapped around the outer portion of the housingmeans.
 14. The apparatus of claim 12 wherein the coil of electricallyconductive material comprises a planar helix.
 15. The apparatus of claim1 and further comprising:means encapsulating the housing means, theencapsulating means being totally implantable within the aforesaidcavity; and a fluid disposed within the encapsulating means betweeninterior walls of said encapsulating means and the housing means. 16.The apparatus of claim 15 and further comprising collar means formounting the encapsulating means within the aforesaid cavity, theencapsulating means and the collar means having engaging portions formedrespectively thereon, the encapsulating means being movable within thecollar means to a desired location therein.
 17. The apparatus of claim16 wherein the engaging portions on the encapsulating means and on thecollar means comprise mating threads.
 18. The apparatus of claim 17 andfurther comprising slot means in at least the upper portions of theencapsulating means to facilitate rotation of the encapsulating meanswithin the collar means.
 19. The apparatus of claim 16 wherein theencapsulating means has a portion thereof which is structurallyrelatively thin to facilitate transfer of pressure externally imposed onthe encapsulating means to the fluid disposed within the encapsulatingmeans and externally of the housing means.
 20. The apparatus of claim 1wherein the housing has an aperture therein used for sealing the mass offluid within the chamber, the apparatus further comprising means forsealing the aperture to prevent introduction of extraneous gaseousmaterial into the chamber, the last-mentioned means comprising pin meansinsertable into the aperture, and solder means for joining the pin meansto a portion of the exterior surface of the housing.
 21. The apparatusof claim 20 wherein the last-mentioned means comprise a collar of metalformed about the periphery of the aperture, the solder means joining tothe collar.
 22. Apparatus for sensing pressure within a cavity in thebody of a living entity, comprising:means deformable in respone topressure within the cavity, said means comprisinga first housing, asecond housing disposed within the first housing, thereby to definefirst and second chambers within said first housing between interiorwalls of the first housing and exterior walls of the second housing, thesecond housing defining an interior chamber therewithin and furtherhaving an aperture communicating the interior chamber with one of theaforementioned chambers, a fluid disposed within the aforesaid chambers,and, pressure responsive means carried on the second housing andextending into the interior chamber, the pressure responsive means beingdeformable by pressure imposed on the apparatus to change the volumeoccupied by the mass of fluid within the interior chamber; and, circuitmeans for absorbing electromagnetic radiation imposed thereon fromexternally of the circuit means at frequencies indicative of thedeformation of the first-mentioned means.
 23. The apparatus of claim 22and further comprising at least two electrically conductive surfacesdisposed in opposing relation to each other and movable relative to eachother in response to deformation of the pressure responsive means, thesurfaces forming at least a capacitive portion of the circuit means. 24.The apparatus of claim 22 wherein the pressure responsive means compriseat least one bellows formed of electrically conductive material andhaving a substantially planar end portion extending into the interiorchamber, the interior of the bellows communicating with the one of thefirst or second chamber which does not communicate with the interiorchamber through the aforementioned aperture.
 25. The apparatus of claim24 and further comprising:electrically conductive means disposed withinthe interior chamber and having a substantially planar surface opposingthe planar end portion of the bellows said surface and said planar endportion being movable relative to each other in response to deformationof the bellows.
 26. The apparatus of claim 24 and further comprising acoil of electrically conductive material, the coil being electricallyconnected to the bellows and to the electrically conductive means. 27.The apparatus of claim 26 wherein the coil is wrapped about an outerportion of the second housing means.
 28. The apparatus of claim 22 andfurther comprising inductive means disposed in proximity to theelectrically conductive surfaces, the inductive means forming aninductive portion of the circuit means.
 29. The apparatus of claim 28wherein the inductive means comprise a coil of electrically conductivematerial.
 30. The apparatus of claim 28 wherein the coil is electricallyconnected to each of the electrically conductive surfaces to form aresonant circuit.
 31. The apparatus of claim 22 wherein the fluidcomprises silcone liquid.
 32. The apparatus of claim 22 and furthercomprising:means housing the first-mentioned means and the circuitmeans, the housing means being totally implantable within the aforesaidcavity; means for imposing electromagnetic radiation on the circuitmeans within the implanted housing means; and, means for detecting thefrequency at which the circuit means most efficiently absorbs theimposed electromagnetic radiation.
 33. The apparatus of claim 22 andfurther comprising:means for converting the detected frequency to acorresponding pressure reading and for displaying the pressure reading.34. Apparatus for sensing pressure within the cranial cavity of a livingentity, comprising:a first housing; a second housing disposed within thefirst housing, thereby to define first and second chambers within saidfirst housing between interior walls of the first housing and exteriorwalls of the second housing, the second housing defining an interiorchamber therewithin and further having an aperture communicating theinterior chamber with one of the aforementioned chambers; a fluiddisposed within the interior chambers and within the first and secondchambers; sealing means for sealing the first chamber from communicationwith the second chamber; and, pressure responsive means carried on thesecond housing and extending into the interior chamber, the pressureresponsive means being deformable by pressure imposed on the apparatusto reduce the mass of fluid within the interior chamber.
 35. Apparatusfor sensing pressure within the cranial cavity of a living entity,comprising:a housing formed of non-porous and electricallynon-conductive material, the housing defining a chamber; a predeterminedmass of fluid disposed within the chamber; pressure responsive meanscarried on the housing and extending into the chamber, said pressureresponsive means being formed of non-porous and elastically compliantmaterial and being deformable by pressure imposed on the apparatus toreduce the volume of the mass of fluid within the chamber; and, circuitmeans for absorbing electromagnetic radiation imposed thereon fromexternally of the apparatus at frequencies indicative of the deformationof the pressure responsive means, the circuit means including at leasttwo electrically conductive surfaces at least one of which is carried onthe pressure responsive means, the conductive surfaces being disposed inopposing relation to each other and movable relative to each other inresponse to deformation of the pressure responsive means, the surfacesforming at least a capacitive portion of the circuit means, the circuitmeans also including inductive means disposed in proximity to theelectrically conductive surfaces, the inductive means forming aninductive portion of the circuit means, the inductive means beingelectrically connected to each of the electrically conductive surfacesto form a resonant circuit.